Complexation of lead(II) by chlorogenic acid: experimental and theoretical study.

Density functional theory (DFT) structure calculations and time-dependent DFT electronic excitation calculations have been performed on chlorogenic acid (H(3)CGA), a polyphenolic compound, used as a model molecule of humic substances. The different deprotonated forms of H(3)CGA have also been investigated. H(3)CGA is a multisite ligand that presents several metal complexing sites in competition, notably the carboxylic and catechol moieties. In low acidic aqueous medium, the complexation of Pb(II) has been followed by electronic absorption spectrometry. The formation of two complexes of stoichiometry metal:ligand 1:1 (log beta(1:1) = 3.39) and 2:1 (log beta(2:1) = 7.12) has been highlighted with use of chemometric methods. The theoretical spectrum of the 1:1 complex obtained by TD-DFT methodology shows the formation of a chelate [Pb(H(2)CGA)(H(2)O)(3)](+) with the metal fixation at the level of the carboxylate function. The second complexing site, the catechol moiety, is rapidly involved in the formation of the 2:1 complex from molar ratios [metal]/[ligand] higher than 0.1. The electronic transitions calculated for both free ligand and complexes involved the same molecular orbitals, and no ligand-metal or metal-ligand charge transfer is observed.

Spectroscopic and theoretical studies of the Zn(II) chelation with hydroxyflavones.

The Zn(II) complexation of three naturally occurring organic compounds (3-hydroxyflavone, 5-hydroxyflavone, and 3',4'-dihydroxyflavone) has been investigated by electronic spectroscopy combined with quantum chemical calculations. These three ligands, which differ in the nature of their chelating site, lead to the formation of a complex of 1:1 stoichiometry. The experimental results show that it is possible to class the three studied sites, according to their chelating power toward Zn(II), in the following way: alpha-hydroxy-carbonyl > beta-hydroxy-carbonyl > catechol. Time-dependent density functional theory (TD-DFT) calculations were performed to obtain the excitation energies and oscillator strengths of the different complexes. Several effective core potentials (Los Alamos and Stuttgart/Dresden) were used for the description of the Zn ion. Calculations were also performed without any pseudopotential, and they give very satisfying results in the simulation of UV-vis spectra of the three complexes. Only the MWB28 ECP leads globally to a good quality description of the spectral features, roughly comparable to that obtained when the 6-31G(d,p) basis set is used to describe the Zn(II) orbitals. The analysis of the results shows that the nature of electronic transitions involved in the UV-vis spectra greatly differs according to the substitution pattern of the flavonoid.